1. Field of the Invention
This invention relates to document image processing, and in particular, it relates to image enhancement for document image reproduction.
2. Description of Related Art
During document image reproduction, typically via printing and imaging (PAI), image quality is inevitably degraded. For example, dark texts in the document become lighter after each PAI process. In many cases, it is desirable to maintain visually conspicuous quality characteristics of a document image during its reproduction. One method to improve image quality is to enhance edges in the image; an example is described in U.S. Pat. No. 5,825,937. Another approach is to sharpen the image by contrast mapping; an example is described in U.S. Pat. No. 6,731,821. However, one significant disadvantage of these enhancement methods, when used in PAI for a document image, is that it makes the enhanced content fatter (e.g., text strokes become thicker) as the enhanced edges have greater impact on the neighboring background pixels during printing and imaging.
In many document images, text constitutes a majority of their content. When strokes of a character become thicker, many image features of the text characters, such as side profiles, zoning profile and topology etc., can be impacted. This may have a negative impact on the performance of document image processing systems, such as Optical Character Recognition (OCR) and document authentication systems.